Influenza A virus ts mutants and recombinants were evaluated in vitro and in experimental animals for evidence of growth restriction and genetic stability. Antigenicity and protective effective were also studied in vivo. New ts mutants were produced and characterized genetically. These and the vaccine candidate ts-1(E) and ts-1A2 recombinants are being genotyped by polyacrylamide gel electrophoresis. The ts genes in the A/Udorn/72 ts-1A2 recombinants were shown to effect a predictable level of attenuation when transferred into recombinants bearing the surface antigens of A/Victoria/75(H3N2), A/Alaska/77(H3N2) and A/Hong Kong/77 (H1N1). Thus, influenza viruses belonging to different subtypes can be satisfactorily attenuated by the acquisition of the ts-1A2 mutations. Furthermore, the resulting ts-1A2 recombinants exhibit a remarkable degree of genetic stability in vivo.